The present invention relates generally to methods and systems for suppressing polarization hole burning in rare-earth doped fiber amplifiers. More particularly, the present invention relates to methods and systems for suppressing polarization hole burning using acousto-optic modulation to vary a state of polarization of an input signal.
Long distance optical communication systems have been known to suffer from various polarization dependent effects that may cause a signal-to-noise ratio of the system to lessen. Polarization hole burning (PHB) is one of the polarization dependent phenomena that can severely impair the performance of erbium-doped fiber amplifiers (EDFAs) located in optical fiber communication systems. PHB occurs when a strong, polarized optical signal is launched into an EDFA and causes anisotropic saturation of the amplifier. This effect, which is related to the population inversion dynamics of the EDFA, depresses the gain of the EDFA for light with the same polarization as the saturating signal. Thus, PHB causes a signal having a state of polarization (SOP) orthogonal to the saturating signal to have a gain greater than that of the saturating signal.
In a chain of saturated EDFAs, amplified spontaneous emission (ASE) noise can accumulate faster in the polarization orthogonal to a saturating information signal than along the polarization parallel to the signal. ASE orthogonal to a saturating signal will accumulate at each amplifier stage of the transmission line. The build-up of orthogonal ASE reduces the signal-to-noise ratio (SNR) of the optical transmission system, thus causing possible errors in the received data stream. Accordingly, it is desirable to reduce the effects of PHB in amplified systems in order to maintain a system with good SNR characteristics.
Operating EDFAs in gain compression helps to cause the undesired PHB effect. The degree of gain compression Cp indicates the difference of gain of the amplifier in its operative condition of propagation of a signal with low optical power (i.e., a non-saturating signal experiencing maximum gain, called xe2x80x9cGoxe2x80x9d) with respect to the value experienced by the optical signal in the power level condition at which it is operating (G). An amplifier""s operating gain in decibels can be measured with a saturating signal of input power Si as the following:
G=Soxe2x88x92Si, xe2x80x83xe2x80x83(1) 
where So is the saturated output power. Accordingly, the amount of gain compression equals the following:
Cp=Goxe2x88x92G. xe2x80x83xe2x80x83(2) 
The gain in the orthogonal polarization, on the other hand, can be measured using a probe signal with an input polarization orthogonal to the saturating signal as the following:
Poxe2x88x92Pi=G+xcex94G, xe2x80x83xe2x80x83(3) 
Pi and Po being the input and output power of the probe signal. In equation (3), xcex94G corresponds to the PHB value.
Moreover, the amount of PHB increases as the amplifier goes deeper into gain compression. FIG. 1 is a graph of experimental measurements showing the relationship between the amount of gain compression and the amount of PHB in an EDFA. As shown in this graph, the amount of PHB is only about 0.08 dB for a single EDFA that operates with 3 dB of gain compression. However, as the gain compression increases, so does the PHB. When the EDFA operates in a saturated condition with Cp equal to about 9-10 dB, the PHB is more significant and quantifiable at around 0.2 dB per EDFA.
Furthermore, the amount of PHB in an EDFA depends on the degree of polarization (DOP) of the saturating signal passing through the amplifier. FIG. 2 is a graph of experimental results on an EDFA operating at 10 dB of gain compression. As can be seen from this graph of FIG. 2, as the degree of polarization of the saturating signal diminishes from 100%, the variation of gain induced by PHB also diminishes. This fact illustrates that the deleterious effects from PHB may be lessened by varying the state of polarization. PHB can be reduced by scrambling the SOP of the transmitted optical signal at a rate that is much higher than 1/ts, where ts is the anisotropic saturation time. Because an EDFA takes about 0.5 msec to reach a gain stable condition after variation of a signal""s SOP, the signal""s SOP should be scrambled at about 10 kHz or more in order to overcome the PHB phenomenon.
The literature has proposed several arrangements for mitigating PHB effects in optical communication systems. EP 615,356 and U.S. Pat. No. 5,491,576 disclose a technique for reducing nonlinear signal degradation by simultaneously launching two optical signals of different wavelengths, comparable power levels, and substantially orthogonal relative polarizations into the same transmission path. The resulting overall transmitted signal is therefore essentially unpolarized, and the impact of detrimental polarization dependent effects within the transmission system are reportedly minimized. The combined signal is modulated by a polarization independent optical modulator so that both wavelength components of the combined signal carry the same data, or each wavelength path is separately modulated prior to their combination. Similar disclosure of a system that launches two signals of different wavelengths can be found in Bergano et al., xe2x80x9cPolarization Hole-Burning in Erbium-Doped Fiber-Amplifier Transmission Systems,xe2x80x9d ECOC ""94, pp. 621-628.
U.S. Pat. No. 5,107,358 describes a method and apparatus for transmitting information and detecting it after propagation through a waveguide by means of a coherent optical detector. In particular, FIG. 3 shows a transmitter comprising an optical source generating a single carrier signal which is fed to a modulator. An optical splitter generates two versions of the modulated signal. The first version is fed to a first polarization controller, while the second version is fed via a frequency shifting circuit to a second polarization controller. The polarization of this signal is adjusted by the second controller to be orthogonal to the polarization of the signal from the first controller. The orthogonally polarized signals are then combined by a polarization selective coupler for transmission.
It should be understood that in all the examples described in the ""358 patent, the two optical carrier frequencies will typically be separated by two to three times the bit rate in Hertz. Applicants have observed that by superposing an optical signal with a version of the same having orthogonal polarization and being shifted in frequency by two to three times the bit rate, an optical signal with a bandwidth of the same magnitude (two to three times the bit rate) is obtained. The bandwidth of the filters to be used at the receiver must be equal to or greater than the signal bandwidth. Due to this large filter bandwidth, the noise at the receiver, in the case of a long distance amplified optical telecommunication system, would be too high to allow a good signal reception, particularly for a bit rate greater than 1 Gbit/s.
It is also known from, for example, U.S. Pat. No. 5,327,511 and Heismann et al., xe2x80x9cElectro-optic polarization scramblers for optically amplified long-haul transmission systems,xe2x80x9d ECOC ""94, pp. 629-632, to generate a carrier signal having a single wavelength, modulate the carrier signal with data, and then send the modulated carrier signal through a polarization modulator or scrambler to help alleviate the effects of polarization hole burning. These documents disclose the use of a lithium niobate-based electro-optic modulator with a single path for passing the carrier wavelength and modulating its polarization at, for example, modulation frequencies of 40 kHz and 10.66 GHz. These polarization modulators or scramblers create highly randomized polarization states for the signal. Such devices affect the output polarization according to a control signal and use relatively high levels of power.
From Electronics Letters, Vol. 30, No. 18, p. 1500-1501, Sep. 1, 1994 an acousto optical Ti:LiNbO3 device is known whose transducer is placed at ⅓ of the interaction length, which forms a polarization-independent optical depolarizer consisting of two or more sections of a wavelength tunable TE-TM converter, suitable to suppress polarization hole-burning in EDFAs. The authors present a double stage depolarizer with a xe2x89xa60.03 residual degree of polarization.
As well, acousto-optical waveguide devices are known that provide a polarization rotation to an input optical signal and modulate the signal with an acoustic wave from a modulation source. Relevant publications include, for example, EP 737,880, EP 757,276 and M. Rehage et al., xe2x80x9cWavelength-Selective Polarisation Analyser with Integrated Ti:LiNbO3 Acousto-Optical TE-TM Converter,xe2x80x9d Electronics Letters, vol. 30, no. 14, Jul. 7, 1994.
Applicants have found that the known techniques for minimizing polarization hole burning using electro-optic modulators to rotate the polarization of a carrier signal require undesirably high levels of power. As well, Applicants have discovered that the known techniques for providing a polarization-rotating signal for an erbium-doped fiber amplifier require a much wider band width than is practically acceptable for a receiver in an optical transmission system. Furthermore, systems employing two sources at different wavelengths are difficult to implement, due to the problems in selecting the sources and in stabilizing their wavelengths. WDM transmission by this system would be very complicated and expensive.
In accordance with the present invention, an optical transmission system has been developed to help reduce polarization hole burning in a rare-earth-doped fiber amplifier by converting an optical carrier signal having a characteristic wavelength into a polarization-rotating optical carrier. The system employs an acousto-optic modulator that modulates a portion of the optical carrier. The acousto-optic modulator causes an orthogonal rotation of the polarization of the portion of the optical carrier. A polarization beam combiner then combines the modulated and orthogonal signal from the acousto-optic modulator with the remainder of the original optical carrier signal to produce a polarization-rotating optical carrier. The polarization-rotating optical carrier is inserted into the optical communication system for eventual use within a rare-earth-doped fiber amplifier.
To obtain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an apparatus for reducing polarization hole burning in a rare-earth-doped fiber amplifier within an optical communication system by converting an optical carrier having a characteristic wavelength and an initial state of polarization into a polarization-rotating optical carrier, includes an acousto-optic modulator and a polarization beam combiner. The acousto-optic modulator has a carrier input optically coupled to receive a first portion of the polarized optical carrier, a modulation input electrically coupled to receive an RF modulation frequency, and a modulator output. The acousto-optic modulator includes circuitry for orthogonally converting polarization of the polarized optical carrier and shifting the polarized optical carrier frequency by the modulation frequency. The polarization beam combiner has a first input optically coupled to receive the orthogonally SOP (State of Polarization) -converted and frequency-shifted polarized signal, a second input optically coupled to receive a second portion of the polarized optical carrier, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
In another aspect, the invention includes an optical transmitter for reducing polarization hole burning in a rare-earth-doped fiber amplifier within an optical communication system having an optical source for transmitting an optical carrier having an initial state of polarization, a splitter, a modulation source for providing a modulation signal, an acousto-optic modulator, an attenuator, and a polarization beam combiner. The splitter is positioned downstream from the optical source, has an input, a first output, and a second output, and divides the optical carrier received at the input between the first output and the second output. The acousto-optic modulator has a carrier input optically coupled to the first output of the splitter, a modulation input electrically coupled to the RF modulation source, and a modulator output. The acousto-optic modulator includes circuitry for orthogonally converting polarization of the optical carrier and frequency shifting the optical carrier by the frequency of the modulation signal. The polarization beam combiner has a first input optically coupled to receive the orthogonally polarization converted and frequency-shifted optical signal, a second input optically coupled to the attenuator, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
In another aspect, the present invention includes a method of suppressing polarization hole burning in a rare-earth-doped fiber amplifier within an optical communication system including the steps of splitting an optical carrier signal into a first sub-carrier signal and a second sub-carrier signal, and rotating orthogonally the polarization of the first sub-carrier signal and modulating the first sub-carrier signal with a RF modulation frequency to create an orthogonal-modulated sub-carrier signal. The method further includes the steps of combining the orthogonal-modulated sub-carrier signal and the second sub-carrier signal to produce a polarization-rotating carrier signal, and passing the polarization-rotating carrier signal downstream In the optical communication system to the rare-earth-doped fiber amplifier.
In a further aspect, the present invention includes an acousto-optic modulator for rotating the polarization of an optical carrier signal, comprising: a substrate of a birefringent and photo-elastic material; a first port on the substrate for receiving the optical carrier signal from an optical waveguide; a splitter having an input coupled to the first port, a first output, and a second output; a first optical waveguide branch coupled at one end to the first output of the splitter; a second optical waveguide branch coupled at one end to the second output of the splitter; an acoustic waveguide on the substrate including at least a portion of the first optical waveguide branch; an acoustic wave generator positioned on the substrate over at least a portion of the acoustic waveguide; and a polarization splitter having a first input coupled to another end of the first optical waveguide branch, a second input coupled to another end of the second waveguide branch, and an output.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of this invention.